Hello, @ChrisSA.
Currently, the ability to manually override the pay period in running your first paycheck for 2020 is not available. You can leave it as is. What is important is the tax year of paycheck date which falls on 2020. This affects the employee's taxable income not the pay period.
If you pay employees on the last day of December for this work, the paycheck date of December 31, 2019, and this would mean the pay is taxable income in 2019. However, since your first paycheck date falls on January 17, 2020, this mean that the employee's pay is taxable income in 2020.
You can read learn more about how the Doctrine of Constructive Receipt affects payroll through this publication: In IRS Publication 334, Tax Guide for Small Business.
Also, I'd suggest consulting with your accountant or other tax professional so you'll be guided accurately with your tax-related tasks.
I always got your back so please don't hesitate to reach out if you have other questions or concerns in running payroll.
